In the classical planar heterojunction perovskite solar cells (PSCs), the electron conducting TiO2 layer shows lower conductivity than the hole transporting materials such as spiro-OMeTAD, which becomes one of the key problems in improving the power conversion efficiency (PCE) of PSCs. In this study, the surface of compact TiO2 layer is modified by a thin self-assembled dodecanedioic acid (DDDA) molecular layer. The TiO2 substrates are immersed into the DDDA solution for 0.5, 2.5, 4.5, 22 h, respectively. It is found that the PCE of PSCs is improved when using the DDDA modified TiO2, showing optimized PCE of 15.35%0.75% under AM 1.5G illumination at 100 mWcm-2 after 4.5 h modification. The short current density (JSC) of the best device is improved from 20.34 mA cm-2 to 23.28 mA cm-2, with the PCE increasing from 14.17% to 15.92%. And it is found that the hysteresis of the PSC is also reduced remarkably with hysteresis index decreasing from 0.4288 to 0.2430. In the meantime, the device with DDDA modification shows a significant improvement in light stability, keeping 71% of its initial PCE value after 720 min exposure under AM 1.5G illumination at 100 mW cm-2 without encapsulation. As a contrast, the device without DDDA modification keeps 59% of its initial PCE value under the same condition. To reveal the mechanism, we investigate the surface energy level change using ultraviolet photoemission spectroscopy. It is found that after DDDA modification, the valence-band maximum energy (EVBM) of TiO2 decreases from -7.25 eV to -7.32 eV, and the conduction-band minimum energy (ECBM) of TiO2 from -4.05 eV to -4.12 eV. The shifting of energy level optimizes the energy level alignment at the interface between the TiO2 and perovskite. It promotes the transport of electrons from perovskite layer to compact TiO2 layer and obstructs the transport of holes from perovskite layer to compact TiO2 layer more effectively. In addition, the decrease of ECBM implies the increase of conductivity of TiO2. We further design a series of electrical experiments, and confirm that the modification improves the conductivity of TiO2 obviously with both contact resistance and thin-film resistance decreasing. In summary, our results indicate the enormous potential of the compact TiO2 layer with a thin self-assembled DDDA molecular layer modification to construct efficient and stable planar heterojunction PSCs for practical applications.